Although their Feb. 9 divorce announcement took many by surprise, pals close to Simpson and husband Pete Wentz tell the new Us Weekly that the spouses of nearly three years "have been talking about splitting up forever."

Simpson, 26, had coped in part with the impending split by having some late, late-night fun. "Ashlee has been out late partying for the past few months," says one insider. Specifically, the singer-actress -- and mom to son Bronx, 2 -- hangs with a crew of pro skateboarders in San Clemente, Calif., and returns home at sunrise.

"She would lose track of time," the insider says.

Fall Out Boy rocker Wentz, 31, "started getting burnt out" by those antics, another source says. On the road with his band, he "would constantly check in on Ashlee, and he'd have his friends call the house and her cell to make sure she said she was where she said she was going to be," an insider explains. "He felt like he couldn't trust her."

For much more on the split -- pals' concern for Simpson's super "skinny" frame, how she's "frustrated and insecure," why they're "still talking" -- pick up the new Us Weekly, on stands now.